The present invention relates to implanted pacemakers, and more particularly to an implanted pacemaker that includes a motion or activity sensor for sensing the physical motion or activity of a patient in whom the pacemaker has been implanted.
Pacemakers are used to provide an electrical stimulus to the heart in the absence of normal heart activity in order to keep the heart beating at a safe level. In turn, a heart that beats at a safe level maintains an adequate supply of blood to the body tissue, thereby providing the needed supply of oxygen to the body cells and removing wastes from the body cells--in short, to keep the body cells alive, and hence to keep the patient alive.
As the physiological activity of the patient increases, many of the body cells must work harder, thereby requiring an increased supply of oxygen and an increased removal of carbon dioxide. (Oxygen is the primary fuel or source of energy that is used by the cells as they perform their body function(s), while carbon dioxide is the primary waste product that is expelled from the cells after their work is done.) In a normal healthy person, this increased supply of oxygen is provided by the heart and/or lungs increasing their respective rates of volumetric flow, i.e., by the heart increasing the rate and/or efficiency with which it pumps the blood through the body, and by the lungs increasing the rate and/or efficiency with which they inhale and exhale oxygen and carbon dioxide.
In some patients with a pacemaker, however, the heart may not be able to respond to a physiological need to pump more blood because of the heart's dependency on a stimulus from the pacemaker in order to beat (contract or depolarize). Accordingly, for these pacemaker patients, there is a need to make the pacemaker sensitive to physiological demands so that the pacemaker-provided stimulus can be provided in accordance with these demands. If this need can not be not met, as has often been the case with prior art pacemakers, then the patient must be cautious and limit his or her physical activity so that the physiological demands are kept within safe limits. Unfortunately, this limitation may severely restrict the physical activity of a pacemaker patient.
Recognizing this need, prior art pacemakers have been developed that are programmable, i.e., the basic rate at which the stimulation pulses are provided by the pacemaker can be noninvasively changed to suit the particular needs of the patient. However, even programming, while extremely useful in many ways, has not been totally satisfactory because it still requires that a programming change be made, and such changes can typically only be made by a physician or other technician having the proper equipment. Moreover, even if the patient has access to the proper programming equipment, the patient can not always know when his or her physiological demands will be changing. Hence, there is a need in the art to provide a pacemaker that automaticallv responds to the physiological demands of the patient so that the needed pacemaker-provided stimuli can be provided at the appropriate times and at the appropriate rates.
Automatic physiologically responsive pacemakers are known in the art. Such pacemakers have relied on numerous and varied sensed parameters as a physiological indicator that the demands of the patient are changing. For example, it is known in the art to measure blood temperature (see U.S. Pat. No. 4,436,092), blood oxygen concentration (see U.S. Pat. No. 4,202,339), repolarization interval (see U.S. Pat. No. 4,228,803), respiration rate (see U.S. Pat. Nos. 3,593,718 and 4,567,892), minute ventilation (see U.S. Pat. No. 4,596,251), and physical activity as sensed by a piezoeletric element (see U.S. Pat. Nos. 4,140,132 and 4,428,378) as parameters that indicate a change in physiological need. Further, the applicant is aware of proposals to measure the depolarization interval (see pending U.S. application Ser. No. 716,831, filed 03/27/85) and to use various mechanical devices in conjunction with acoustic sensing equipment in a pending German application in order to determine the physiological demands of a patient so that the stimulation rate of the pacemaker can be adjusted accordingly.
For purposes of the present invention, it is the physical activity of the patient, as sensed by measuring the motion or movement of the patient, that comprises the physiological parameter to be used for controlling the rate of a pacemaker.
As indicated above, some attempts are known in the art for causing a pacemaker to sense and respond to physical activity. Using a piezoelectric element, as is taught in the U.S. Pat. No. 4,428,378 for example, requires that the electrical analog signal from the piezoelectric element be processed in a prescribed manner. While such processing can be done, it requires special filtering and thresholding circuitry, all of which adds to the bulk and power consumption of the pacemaker. Needless to say, keeping power consumption and size to a minimum is a primary goal of all implantable pacemaker design. Hence, any added circuits which tend to increase the size, bulk, or power consumption of a pacemaker are disfavored.
Further, there are other disadvantages to using a piezoelectric element as a sensor of physical activity. For example, the physical construction of a piezoelectric element makes it somewhat direction dependent. Hence, depending upon how it is oriented within the patient, it may be less sensitive to physical movement in a given direction (X, Y or Z axis) than to movement in another direction. Further, whenever an analog signal is sensed, such as the signal from a piezoelectric element, it usually must eventually be converted to some sort of digital signal that can interface with the basic digital circuits used to realize modern pacemaker circuits. While analog-to-digital circuits are well known in the art, they too add to the bulk and power consumption of the pacemaker.
Where an acoustic pickup device is employed in conjunction with a mechanical device, which mechanical device is designed to generate various sounds as a function of physical activity, such as is disclosed in the above-mentioned German patent application, an analog-to-digital conversion must still occur. Further, an added element (the mechanical device that serves as the source of the acoustic signal and/or the microphone pickup element) must be included within the pacemaker.
Accordingly, what is needed is a way of detecting physical activity or body motion using a simple detector device that can interface directly with the digital circuits of the pacemaker and that does not noticeably add to the complexity, bulk, or power consumption of the pacemaker.
Finally, it is noted that even though a dual chamber pacemaker (i.e., one that can provide stimulation pulses to both chambers of the heart) may theoretically be operable in a mode that is responsive to the physiological demands of some patients, there may be practical reasons why such a dual chamber pacemaker is not used. For example, in a patient with complete heart block, a dual chamber pacer operating in the DDD mode of operation (i.e., the pacemaker paces in both the atrium and ventricle, and senses in both the atrium and ventricle) will respond to the heart's natural pacemaker--the SA (Sinoatrial) Node. This occurs because the atrium responds to the SA Node and causes the atrium to contract. The atrial sensing circuits of the DDD pacemaker sense this contraction and, after an appropriate AV delay, generate a ventricular stimulation pulse that causes the ventricle to contract. Thus, the DDD pacer guarantees rate responsiveness and AV synchrony. However, as indicated, there may be some circumstances where a DDD pacemaker would not be used. Hence, for these patients, there is still a need for a single chamber pacemaker that is automatically responsive to the patient's physiological needs.
In the description of the invention that follows, it is noted that in general no distinction will be made between whether a single chamber or a dual chamber pacemaker is used. This is because the motion or activity sensor described herein can be used with either type of pacemaker.